Evaluation of Pathogenicity of Symbiotic Bacteria of Entomopathogenic Nematodes against the Larvae of Tobacco Caterpillar, Spodoptera litura F. (Lepidoptera; Noctuidae)

Abstract

The tobacco caterpillar, Spodoptera litura is a polyphagous pest with attacking almost 290 host plants species globally causing a wide range of yield losses to the agricultural crops. In this investigation symbiotic bacteria of entomopathogenic nematodes were evaluated for their insecticidal activity against larvae of S. litura under lab conditions. The intact cell suspension and cell-free extract were screened for their bacterial pathogenicity and the results showed mortality of larvae after 24h of post treatment. The percent mortality treated with intact cell suspension was significantly higher in larvae treated with EPB3 strain with 90% which was on par with the reference strain Bacillus thuringiensis after 72h of post treatment. There is no significant difference among the larval mortality treated with cell-free supernatant. The virulence test was carried out to find out the LD50 and LT50 against the larvae and the results showed that EPB3 and B. thuringiensis required 104 CFU for 50% lethality of larvae with a media lethal time of 24h and 22h, respectively when treated with cell suspension. In case of larvae treated with cell-free extract, LD50 value with 105-106 CFU with a median lethal time ranging from 33-45h. The results showed that the isolates EPB3 (Xenorhabdus nematophilus) is comparatively more virulent than other Photorhabdus isolates against the test insect S. litura. Further these symbiotic bacteria can be screened for their biocontrol efficiency under greenhouse and field conditions and can be developed as potential biopesticides against the insect pests.